Nanofinishing - Textile Document

Another limitation relaxed by some advanced finishing processes using loose abrasives is to finish complicated geometries by enhancing reach of abrasive particles to difficult-to-access

Nano-Finishing Techniques

Sunil Jha and V K Jain

Department of Mechanical Engineering

Indian Institute of Technology Kanpur - 208016, INDIA

in conventional processes would reach 1 µm, while in precision and ultraprecision machining would reach 0.01µm (10 nm) and 0.001µm (1 nm) respectively [2] His predictions made around two decades before are in line with the current advances in manufacturing technology These accuracy targets for today’s ultraprecision machining can’t be achieved by simple extension of conventional machining processes and techniques

Fig 1 Achievable Machining Accuracy [2]

Nanotechnology [2] was first used to classify integrated manufacturing technologies and machine systems which provide ultraprecision machining

Pre isio Ma hinin

Ulra Pre isio Ma hinin

Atomic Lattice Separation

Turning & Milling Machines

capabilities in the order of 1 nm Since then ultraprecision technologies have grown rapidly over recent years and have tremendous impact on the development of new products and materials Nanotechnology is the target of ultraprecision machining because the theoretical limit of accuracy in machining of substance must be the size of

an atom or molecule of the substance With the advent of new materials, manufacturing is facing challenges in machining them to meet their functional requirements As the demand moves from the microtechnology (1µm accuracy capability) to the nanotechnology region (1 nm accuracy) the systems engineering demands rapid increase in stringency and complexity [2] The traditional finishing processes alone are therefore incapable of producing required surface characteristics

to meet the demand of nanotechnology Even in certain cases these processes can be used but then they require expensive equipments and large labour, finally leave them economically incompetent

New advanced finishing processes were developed in last few decades to overcome limitations of traditional finishing processes in terms of higher tool hardness requirement and precise control of finishing forces during operation This helped in finishing harder materials and exercising better in process control over final surface characteristics Another limitation relaxed by some advanced finishing processes using loose abrasives is to finish complicated geometries by enhancing reach of abrasive particles to difficult-to-access regions of the workpiece surface In this way, newly developed finishing processes are to a large extent helpful in meeting requirements of 21st century manufacturing

Before continuing discussion on advanced ultra precision finishing processes, it is useful to understand the principle of action of commonly used traditional finishing processes – grinding, lapping and honing All these processes use multipoint cutting edges in the form of abrasives, which may or may not be bonded, to perform cutting action These processes have been in use from the earliest times because of their capability to produce smooth surface at close tolerances Higher hardness of abrasive particles is an important prerequisite for processing If properly conducted these abrasive machining processes can produce a surface of higher quality with a controlled surface roughness combined with a desirable residual stress distribution and freedom from surface and sub-surface damages [3]

Grinding is the most widely used abrasive finishing process among all traditional processes used in production In grinding the material is removed from the workpiece surface by relative motion of the cylindrical wheel having abrasive particles embedded on its periphery The abrasive particles are bonded together to form porous revolving body [4] which when come in contact with workpiece results in material removal Grinding in a broad perspective is mainly divided into two regimes – stock removal grinding (SRG), and form and finish grinding (FFG) In SRG, the main objective is to remove the superfluous material from the surface, while in FFG, the surface quality is main concern The abrasives on a grinding wheel are firmly bonded with an appropriate binder and at the same time also have possibility to allow grain fracture to renew cutting edges Abrasive grain wears rapidly on grinding harder materials so a less strongly bonded wheel is preferred for operation Porosity on the grinding wheel is a controllable factor to provide rooms to accommodate chips

Wheels bonded with glass are strongest and hardest while organic bonds are of lower strength The size and distribution of grits along with wheel structure play an important role in grinding performance A proper selection of wheel according to finishing requirements is very important The application of grinding is mainly available for simple geometries like cylindrical or plane surface where size is limited

by grinding wheel movement

Lapping uses loose abrasives to finish the surface It works on three body abrasive wear principle in which finishing action takes place through abrasion by hard particles trapped between workpiece surface and a relatively soft counter formal surface called lap After introducing abrasive slurry between workpiece and lap surface, the workpiece is held against lap and moved in random paths under pressure Simple three dimensional shapes and curved surfaces (concave, convex etc.) to some extent can be finished by designing compliant lap As this process is generally employed for improving surface finish and accuracy, the amount of material removed is insignificant

Honing is another abrasive finishing process generally used to finish internal cylindrical surfaces The abrasives in the form of stones or sticks carried in an expanding and oscillating mandrel are used to generate random cross-marked surface with good finish The stick pressure on workpiece surface is comparatively more than lapping The surface produced after honing has self-lubricating property due to oil retaining capability in cross-hatched pattern

There are many advances taking place in the finishing of materials with fine abrasives, including the processes, the abrasives and their bonding, making them capable of obtaining nanometer order surface finish Earlier there has been a limit on the fine size of abrasives (~a few µm) but today, new advances in materials syntheses have enabled production of ultra fine abrasives in the nanometer range Abrasives are used in a variety of forms including loose abrasives (polishing, lapping), bonded abrasives (grinding wheels), and coated abrasives for producing components of various shapes, sizes, accuracy, finish and surface integrity

The electronics and computer industries are always in demand of higher and higher precision for large devices and high data packing densities The ultimate precision obtainable through finishing is when chip size approaches atomic size (~ 0.3 nm) [2] To finish surfaces in nanometer range, it is required to remove material in the form of atoms or molecules individually or in the groups Some processes like Elastic Emission Machining (EEM) and Ion beam Machining (IBM) work directly by removing atoms and molecules from the surface Other processes based on abrasive wear remove them in clusters On the basis of energy used, the advanced finishing processes (AFPs) can be broadly categorized into mechanical, thermoelectric, electrochemical and chemical processes [5] The performance and use of certain specific process depend on workpiece material properties and functional requirement

of the component In mechanical AFPs, very precise control over finishing forces is required Many newly developed AFPs make use of magnetic/electric field to externally control finishing forces on abrasive particles To name a few, these

magnetic field assisted finishing processes include Magnetic Abrasive Finishing (MAF), Magnetic Float Polishing (MFP), Magnetorheological Finishing (MRF), and Magnetorheological Abrasive Flow Finishing (MRAFF) Chemo Mechanical polishing (CMP) utilizes both mechanical wear and chemical etching to achieve surface finish of nanometer and planarization CMP is the most preferred process used

in semiconductor industry for silicon wafer finishing and planarization Since the material removal in fine abrasive finishing processes is extremely small, they can be used successfully to obtain nanometer surface finish, and very low value of dimensional tolerances Advanced abrasive finishing processes belong to a subset of ultra precision finishing processes which are developed for obtaining nanometer order surface finish A comparison of surface finish obtainable from different finishing process is given in Table 1 This chapter discusses about the principles of working and potential applications of such processes in following paragraphs

Table 1: Comparison of surface finish obtainable by different finishing processes

4 Abrasive Flow Machining (AFM) with SiC abrasives

Hardened steel 50

5 Magnetic Abrasive finishing (MAF) Stainless steel 7.6

6 Magnetic Float Polishing (MFP) with CeO2

7 Magnetorheological Finishing (MRF) with CeO2

Flat BK7 Glass 0.8

8 Elastic Emission Machining (EEM) with ZrO2 abrasives

Silicon <0.5

9 Ion Beam Machining (IBM) Cemented carbide 0.1

Abrasive Flow Machining (AFM) was identified in 1960s as a method to deburr, polish, and radius difficult to reach surfaces and edges by flowing an abrasive laden viscoplastic polymer over them It uses two vertically opposed cylinders, which extrude an abrasive medium back and forth through passage formed by the workpiece and tooling Abrasion occurs wherever the medium passes through the restrictive passages The key components of AFM are the machine, the tooling, types of abrasives, medium composition and process settings [6] Extrusion pressure, number

of cycles, grit composition and type, and fixture design are the process parameters that have the largest impact on AFM results

Fig.2: Abrasive Flow Machining, (a) forces acting on abrasive particle in AFM

process, (b) Experimental setup Abrasive action accelerates by change in the rheological properties of the medium when it enters and passes through the restrictive passages [6] The viscosity of polymeric medium plays an important role in finishing operation [7] This allows it to selectively and controllably abrade surfaces that it flows across The work piece held

by fixture is placed between two medium cylinders which are clamped together to seal so that medium does not leak during finishing process The three major elements

of the process are:

(a) The Tooling, which confines and directs the abrasive medium flow to the areas

where deburring, radiusing and surface improvements are desired

(b) The Machine to control the process variables like extrusion pressure, medium

flow volume, and flow rate

(c) The abrasive laden Polymeric Medium whose rheological properties determine

the pattern and aggressiveness of the abrasive action To formulate the AFM medium, the abrasive particles are blended into special viscoelastic polymer, which show change in viscosity when forced to flow through restrictive passages AFM can process many selected passages on a single workpiece or multiple parts simultaneously Inaccessible areas and complex internal passages can be finished economically and productively It reduces surface roughness by 75 to 90% on cast, machined or EDM'd surfaces [8] The same AFM set up can be used to do a variety of jobs just by changing tooling, process settings and if necessary abrasive medium composition Because of these unique capabilities and advantages, AFM can be applied to an impressive range of finishing operations that require uniform, repeatable and predictable results [8] AFM offers precision, consistency, and flexibility Its ability to process multiple parts simultaneously, and finishing inaccessible areas and complex internal passages economically and effectively led to its application in wide range of industries Aerospace, aircraft, medical components, electronics, automotive

Upper Hydraulic Cylinder

Medium Cylinder Workpiece Fixture

Lower Hydraulic Cylinder

Hydraulic Unit

(b)

Silly putty

Axial force Radial

parts, and precision dies and moulds manufacturing industries are extensively using abrasive flow machining process as a part of their manufacturing activities Recently, AFM has been applied to the improvement in air and fluid flow for automotive engine components, which was proved as an effective method for lowering emissions as well

as increasing performance Some of these potential areas of AFM application are discussed below:

AFM process was originally identified for deburring and finishing critical hydraulic and fuel system components of aircraft in aerospace industries With its unique advantages, the purview of its application has been expanded to include adjusting airflow resistance of blades, vanes, combustion liners, nozzles and diffusers; improving airfoil surface conditions on compressors and turbine section components following profile milling, casting, EDM or ECM operations; edge finishing of holes and attachments to improve the mechanical fatigue strength of blades, disks, hubs, and shafts with controlled polished, true radius edges; and finishing auxiliary parts such as fuel spray nozzles, fuel control bodies and bearing components

The surface finish on the cast blades is improved from original 1.75 µm to 0.4 µm

Ra (Fig 3a) Cooling air holes are deburred and radiused in one operation on turbine disks as large as 760 mm in diameter (Fig 3b) It is also used to remove milling marks and improve finish on the complex airfoil profiles of impellers and blades

Intricate intersections can also be finished easily

Since in the AFM process, abrading medium conforms to the passage geometry, complex shapes can be processed as easily as simple ones Dies are the ideal workpiece for the AFM process as they provide the restriction for medium flow, typically eliminating fixturing requirements AFM process has revolutionized the finishing of precision dies by polishing the die surfaces in the direction of material flow, producing a better quality and longer lasting dies with a uniform surface and gently radiused edges The uniformity of stock removal permits accurate 'sizing' of undersized die passages Precision dies are typically polished in 5 to 15 minutes unattended operation

Fig 3: 500X photomicrograph showing complete removal of EDM recast layer, (a) before AFM, (b) after AFM [courtesy: Extrude Hone corporation, USA] The original 2 µm Ra EDM finish is improved to 0.2µm with a stock removal of

25 µm per surface Fig 3 shows the complete removal of EDM recast layer In some cases, the tolerances have been achieved as 13 µm [9]

AFM process is used to enhance the performance of high-speed automotive engines It is a well-known fact in the automobile engineering that smoother and larger intake ports produce more horsepower with better fuel efficiency But it's very difficult to obtain good surface finish on the internal passageway of intake ports because of its complex shape [10] The demand for this process is increasing rapidly among car and two wheeler manufacturers as it abrades smoothes and polishes the surfaces of 2- stroke cylinders and 4- stroke engine heads for improved air flow and better performance The AFM process can polish anywhere that air, liquid or fuel

recast layer

flows Rough, power robbing cast surfaces are improved from 80-90% regardless of surface complexities

Advanced high-pressure injection system components in diesel engines are subjected to repeated pulses of very high pressure that can generate fatigue failures at high stressed areas Smoothing and removing surface stress risers, cracks, as well as uniform radiusing of sharp edges by AFM process can significantly extend component life Flow tuned spray orifices of fuel injector nozzles can reduce particulate emissions and improves the fuel efficiency of diesel powered engines

3.2 Magnetic Abrasive Finishing (MAF)

Magnetic abrasives are emerging as important finishing methods for metals and ceramics Magnetic Abrasive Finishing is one such unconventional finishing process developed recently to produce efficiently and economically good quality finish on the internal and external surfaces of tubes as well as flat surfaces made of magnetic or non-magnetic materials In this process, usually ferromagnetic particles are sintered with fine abrasive particles (Al2O3, SiC, CBN or diamond) and such particles are called ferromagnetic abrasive particles (or magnetic abrasive particles) However, homogeneously mixed loose ferromagnetic and abrasive particles are also used in certain applications Fig 4 shows a Plane MAF process in which finishing action is generated by the application of magnetic field across the gap between workpiece surface and rotating electromagnet pole The enlarged view of finishing zone in Fig

4 shows the forces acting on the work surface to remove material in the form of chips Force due to magnetic field is responsible for normal force causing abrasive penetration inside the workpiece while rotation of the magnetic abrasive brush (i.e North pole) results in material removal in the form of chips

The magnetic abrasive grains are combined to each other magnetically between magnetic poles along a line of magnetic force, forming a flexible magnetic abrasive brush MAF uses this magnetic abrasive brush for surface and edge finishing The magnetic field retains the powder in the gap, and acts as a binder causing the powder

to be pressed against the surface to be finished [11] 3D minute and intricately curved shape can also be finished along its uneven surface Controlling the exciting current of the magnetic coil precisely controls the machining force of the magnetic abrasives on the work piece [12]

Fig 4: Plane Magnetic abrasive finishing

N pole

Equipotential line

F z

Since the magnitude of machining force caused by the magnetic field is very low but controllable, a mirror like surface finish (Ra value in the range of nano-meter) is obtained In MAF, mirror finishing is realized and burrs are removed without lowering the accuracy of the shape These fine finishing technologies using magnetic abrasives have a wide range of applications The surface finishing, deburring and precision rounding off the workpiece can be done simultaneously MAF can be used

to perform operations as polishing and removal of thin oxide films from high speed rotating shafts Shinmura et al [13] have applied MAF to the internal surface of work pieces such as vacuum tubes and sanitary tubes

Fig 5 shows the magnetic abrasive jet finishing of internal surface of a hollow cylindrical workpiece It's a variant of MAF process in which working fluid mixed with magnetic abrasives is jetted into the internal surface of the tube, with magnetic poles being provided on the external surface of the tube [14] The magnetic abrasives

in the jet mixed with fluid are moved to the internal surface by magnetic force, where the magnetic abrasives finish the internal surface effectively and precisely

Fig 5: Magnetic abrasive jet finishing Fig 6 shows a schematic of a typical MAF process in which the workpiece to

be machined is located between two magnetic poles The gap between the workpiece and the pole is filled with a magnetic abrasive powder The magnetic abrasive grains are linked to each other magnetically between the north and south magnetic poles along the lines of magnetic force, forming a flexible 2-5 mm long magnetic brush MAF uses this magnetic abrasive brush for surface and edge finishing The magnetic field retains the powder in gaps, and acts as a binder causing the powder to be pressed against the surface to be finished [15] A rotary motion is provided to cylindrical workpiece, such as ceramic bearing rollers between magnetic poles Also axial vibratory motion is introduced in the magnetic field by the oscillating motion of magnetic poles to accomplish surface and edge finishing at faster rate and better quality

The process is highly efficient and the removal rate and finishing rate depends on the workpiece circumferential speed, magnetic flux density, working clearance, workpiece materials, and size, type and volume fraction of abrasives The exciting current of the magnetic coil precisely controls the machining force transferred through magnetic abrasives on the work piece

Nozzle

Pressure Gauge

Abrasive Tank

N

S

+ _

Compressed Air

Magnetic Pole

Work Piece

Stabilized DC Power

Magnetic Abrasives

Fig 6: Magnetic Abrasive Finishing of cylindrical surface Since the magnitude of machining force caused by the magnetic field is very low,

a mirror like surface finish (Ra value in the range of nano-meter) can be obtained Results were reported in the literature of finishing stainless steel rollers using MAF to obtain final Ra of 7.6 nm from an initial Ra of 0.22 µm in 30 seconds [16]

3.3 Magnetorheological Finishing (MRF)

Traditional methods of finishing high precision lenses, ceramics and semiconductor wafers are very expensive and labor intensive The primary obstacle in manufacturing high precision lenses is that lenses are usually made of brittle materials such as glass, which tends to crack while it is machined Even a single microscopic crack can drastically hinder a lens's performance, making it completely ineffective for its intended application Every device that uses either lasers or fiber optics requires at least one high precision lens, increasing its demand higher than ever

Lens manufacturing can be classified into two main processes: grinding and finishing Grinding gets the lens close to the desired size, while finishing removes the cracks and tiny surface imperfections that the grinding process either overlooked or created The lens manufacturer generally uses its in-house opticians for the finishing process, which makes it an arduous, labor-intensive process Perhaps the biggest disadvantage to manual grinding and finishing is that it is non-deterministic To overcome these difficulties, Center for Optics Manufacturing (COM) in Rochester, N.Y has developed a technology to automate the lens finishing process known as Magnetorheological Finishing (MRF) [17]

The MRF process relies on a unique "smart fluid", known as Magnetorheological (MR) fluid MR-Fluids are suspensions of micron sized magnetizable particles such as carbonyl iron, dispersed in a non-magnetic carrier medium like silicone oil, mineral oil or water In the absence of a magnetic field,

an ideal MR-fluid exhibits Newtonian behaviour On the application of an external magnetic field to a MR-suspension, a phenomenon known as Magnetorheological

Electromagnet Magnetic

abrasive brush

Cylindrical

workpiece

effect, shown in Fig.7, is observed Fig 7a shows the random distribution of the

particles in the absence of external magnetic field; In Fig 7b, particles magnetize

and form columns when external magnetic field is applied The particles acquire

dipole moments proportional to magnetic field strength and when the dipolar

interaction between particles exceeds their thermal energy, the particles aggregate

into chains of dipoles aligned in the field direction Because energy is required to

deform and rupture the chains, this micro-structural transition is responsible for

the onset of a large "controllable" finite yield stress [18] Fig 7c shows an

increasing resistance to an applied shear strain, γ due to this yield stress When the

field is removed, the particles return to their random state and the fluid again

exhibits its original Newtonian behaviour

Fig 7: Magnetorheological effect, (a) MRP-fluid at no magnetic field, (b) at

magnetic field strength H, and (c) At magnetic field H & applied shear strain γ

Rheologically, the behaviour of MR-fluid in presence of magnetic field is

described by Bingham Plastic model [19]:

τ = τo + ηγ.

where, τ is the fluid shear stress, τo is magnetic field induced yield shear stress, η

is dynamic viscosity of MR-fluid and γ& is Shear rate [s-1]

The dynamic viscosity is mostly determined by the base fluid The field

induced shear stress τo depends on the magnetic field strength, H The strength of

the fluid (i.e the value of static yield shear stress) increases as the applied

magnetic field increases However, this increase is non-linear since the particles

are ferromagnetic and magnetizations in different parts of the particles occur

non-uniformly [20] MR-fluids exhibit dynamic field strength of 50-100 kPa for

applied magnetic fields of 150-250 kA/m (~2-3 kOe) [21] The ultimate strength

of MR-fluid is limited by magnetic saturation

The ability of electrically manipulating the rheological properties of MR-fluid

attracts attention from wide range of industries and numerous applications are

explored [22] These applications are use of MR-fluid in shock absorbers and

damping devices, clutch, brakes, actuators, and artificial joints The magnetic field

applied to the fluid creates a temporary finishing surface, which can be controlled

in real time by varying the field's strength and direction The standard MR fluid

composition is effective for finishing optical glasses, glass ceramics, plastics and

some non-magnetic metals [23]

In the Magnetorheological finishing process as shown in Fig 8, a convex, flat,

or concave workpiece is positioned above a reference surface A MR fluid ribbon

is deposited on the rotating wheel rim, Fig 9 By applying magnetic field in the

γ

H

H Abrasive particles CIPs

Lens

Pole piece

X Moving wall

MRP fluid Polishing spot

Y

gap, the stiffened region forms a transient work zone or finishing spot Surface smoothing, removal of sub-surface damage, and figure correction are accomplished by rotating the lens on a spindle at a constant speed while sweeping the lens about its radius of curvature through the stiffened finishing zone [24] Material removal takes place through the shear stress created as the magnetorheological polishing ribbon is dragged into the converging gap between the part and carrier surface The zone of contact is restricted to a spot, which conforms perfectly to the local topography of the part Deterministic finishing of flats, spheres, and aspheres can be accomplished by mounting the part on rotating spindle and sweeping it through the spot under computer control, such that dwell time determines the amount of material removal

Fig 8: Magnetorheological finishing process

Fig 9: A Vertical MRF Machine [24]

The MR-polishing fluid lap has following merits over traditional lap:-

1 Its compliance is adjustable through the magnetic field

2 It carries heat and debris away from the polishing zone

3 It does not load up as in grinding wheel

Pump Electromagnet

Wheel RotationFluid Delivery

Fluid Pickup

Pump

MR Fluid Conditioner

4 It is flexible and adapts the shape of the part of the workpiece which is in its contact

The computer controlled Magnetorheological finishing process has demonstrated the ability to produce the surface accuracy of order 10-100 nm peak to valley by overcoming many fundamental limitations inherent to traditional finishing techniques [17].These unique characteristics made Magnetorheological Finishing as the most efficient and able process for high precision finishing of optics MRF makes finishing

of free form shapes possible for first time

Applications that use high precision lenses include medical equipment such as endoscopes, collision-avoidance devices for the transportation industry, scientific testing devices and military's night vision equipment like infrared binoculars Missiles are equipped with a wide variety of high precision lenses for navigation, target location, and other functions The nano diamond doped MR fluid removes edge chips, cracks, and scratches in sapphire bend bars

3.4 Magnetorheological Abrasive Flow Finishing (MRAFF)

In AFM, the polishing medium acts as compliant lap and overcomes shape limitation inherent in almost all traditional finishing processes As abrading forces in AFM process mainly depend on rheological behaviour of polymeric medium, which is least controllable by external means, hence lacks determinism The process named as Magnetorheological finishing described above, uses magnetically stiffened ribbon to deterministically finish optical flats, spheres and aspheres In order to maintain the versatility of Abrasive Flow Machining process and at the same time introducing determinism and controllability of rheological properties of abrasive laden medium, a

new hybrid process termed as “Magnetorheological Abrasive Flow Finishing

(MRAFF)” is used, Fig 10 [25] This process relies on smart behaviour of magnetorheological fluids whose rheological properties are controllable by means of external magnetic field

Fig 10: Development of Magnetorheological Abrasive Flow Finishing process [25] The use of magnetorheological polishing fluid with cerium oxide abrasives for finishing optical lenses up to the level of 0.8 nm R.M.S (root mean square) value has already been demonstrated by MRF process [17] MRAFF process has the capability

of finishing complex internal geometries up to nanometer level It imparts better

MRF

Magnetorheological Finishing (Smart Magnetorheological polishing slurry)

AFM

Abrasive Flow Machining (Finishing through flow of abrasive laden medium)

MRAFF

Magnetorheological Abrasive Flow

Finishing (Finishing through flow of smart magnetorheological polishing fluid)